To paraphrase the late, great Jane Austen, it is a truth universally acknowledged that inside every good girl is a bad girl dying to go off on someone, desperate to shake her booty and sing for more than just a mirror, maybe even itching for the chance to drink and smoke and cuss and sleep around, and not necessarily in that order.

That's what Beyoncé Giselle Knowles' husband of nine months, Mr. Jay-Z, discovered not long ago when B., as her friends call her, swaggered home from the set of the film Cadillac Records in a bad mood. Make that a badass mood. B. had been playing the part of the great R&B singer Etta James, a role that gave her license to express a dark side that few even knew she possessed.

As Tina Knowles, Beyoncé's mother and best friend, puts it, "Etta's a little rough around the edges. And when Beyoncé came off the set, she'd be rough too, and a little snappy. Even her walk. She had said, `Mama, show me that Galveston walk.' She modeled it after our housekeeper, Janelle, who's what we call a real beer-joint woman. Smoking, swaggering."

Had she not become one of the world's most successful entertainers—more than 100 million records sold as a solo and group artist and, according to Forbes, some $80 million in earnings in the past year alone for her music, films, endorsements, and fashion line, making her the second highest-grossing pop act after the Police—Beyoncé says she would have gone to college and become a psychologist. "I can instantly see from meeting someone which way they're gonna go," she says this evening in New York City.

She has emerged from behind a partition in a midtown studio, where in remarkably short time she extricated herself from an elaborate space-age outfit made by the London designer Gareth Pugh that she's been wearing for this cover shoot, intended to promote her new album, I Am...Sasha Fierce, and two movies, the aforementioned Cadillac Records and the upcoming Obsessed, a thriller in which she plays a wife whose husband is beset by a stalker—presumably a much greater acting stretch than her role modeled after Motown diva Diana Ross in the 2006 film Dreamgirls.

Now dressed simply in black stretch pants and a button-down, B. turns her analytic instincts on herself. She's trying to understand why it feels so good these days to be so bad. "I'm a people pleaser," she says. "I hold a lot of things in. I'm always making sure everybody is okay. I usually don't rage; I usually don't curse. So for me, it's a great thing to be able to scream and say whatever I want."

This would be true, it seems, even if she has to get a hotel room for the night.

"Listen," she says. "What's that expression? I mean, I feel like you get more bees with honey. But that doesn't mean I don't get frustrated in my life. My way of dealing with frustration is to shut down and to think and speak logically. But playing Etta was a time when I didn't have to do that. I could be loud and let it rip."

It's hard to believe that the low-key, self-possessed woman perched on one end of the couch like a country girl on the edge of a porch is the same strutting diva whose ass-shaking command of the hip-hop stage helped introduce the word bootylicious into the proper pages of the Oxford English Dictionary.

She carries herself in such an unassuming way that she might easily be mistaken for one of the stylists who've spent the day clustered around B., intently grooming for hours a single strand of hair. They could have been scientists conducting experiments on the precise nature of B.'s radiance, which has been a mystery at least since she was seven, when she first mounted a stage at a Houston talent show to sing John Lennon's "Imagine" and proceeded to astonish, even terrify, her parents with her transformation from diffident child to spotlight-hogging entertainer.

"We thought, Who is that? Until then, you didn't even notice when she walked into a room," Tina says.

And to this day, there's the B. who is sitting here chatting like the girl next door and the B. who sashays around the stage with a license to kill, who gets so fired up that she once threw a diamond bangle worth thousands of dollars into the crowd as she was performing (and had to send someone to retrieve it). That she can switch her star power on and off as easily as any idiot flipping a light switch is as evident tonight as it was at the start of her career in Houston back in the 1990s.

"We were really sheltered," B. says of herself and the various girls of Destiny's Child (most notably Kelly Rowland and Michelle Williams), the group she starred with before going on to an imposing solo career that began in 2003 with her CD Dangerously in Love. They might bump and grind like little trollops on stage, but after the show, "we would get on the tour bus and read the Bible," she says. B. didn't go on dates, avoided after-parties. "People always accuse us of being overprotective with Beyoncé," Tina says. "But I was the mother saying, `You gonna go hang out? You gonna go to this party?' She was always just so durn easy, so durn mature."

"There are a lot of things I never did," B. says, "because I believe in watching those true Hollywood stories and I see how easy it is to lose track of your life. Think about Marilyn Monroe."

Her life was the antithesis of those up-from-the-hard-streets sagas glorified in the beat-driven annals of hip-hop. There was no Darwinian ascent from the projects. Her future husband's upbringing at the Marcy Houses in the Bedford-Stuyvesant neighborhood of Brooklyn ( Jay-Z was then known as Shawn Carter) would have been as foreign to her as to white kids listening to rap while mom drives them through the suburbs to tennis lessons. B. came to hip-hop as a performer. It didn't represent a way of life so much as just another musical style. In fact, she originally resisted her role in MTV's 2001 hip-hop version of the opera Carmen, saying, "No way! I can't rap!"

"I grew up upper-class," she says. "Private school. My dad had a Jaguar. We're African-American and we work together as a family, so people assume we're like the Jacksons. But I didn't have parents using me to get out of a bad situation."

In fact, the only way she knew mean streets at all was because her father and manager, Mathew Knowles, then a sales executive at Xerox, took B., her younger sister, Solange, and some friends on a field trip to the Bottoms, a rough section of Houston. "He had my kids talking to the homeless people there," Tina says. "I told him, `Mathew, that could have been dangerous.' He said, `I want them to feel their songs, not just sing them.' "

Encouraged to write her own songs by her father, whom she laughingly says "read too many Berry Gordy books and wanted me to keep the control in the family," B. had little firsthand experience to draw from, given her cosseted life. What did she know about cheatin', lyin', no-good men, that staple of female songcraft? The Knowleses were an upstanding Christian family who attended church regularly, and after services they went to restaurants and amusement parks. A wild time for the kids meant swinging on bed sheets they tied to the upstairs banisters at home.

Like a veteran songwriter, B. learned early on to project herself into lives that weren't her own. She came up with lyrics by eavesdropping as she swept hair off the floor at her mother's beauty salon. "Women in a hair salon are more open than men in a barbershop," B. says. "They'll look at fashion magazines and listen to Anita Baker and talk about men cheatin' on them. That's juicier than any barbershop. I would be listening to what the women were saying, and I would give my two cents on what they should do. I would say, `I think he's lying.' I was just a little girl, but I would say, `That doesn't sound right.' My mom would want to pop me upside my head, and she'd say, `You better get out of here, girl!' "

Durn mature and durn easy B. may have been, but she did on one occasion indulge her inner adolescent diva, which led to what is now known as "the slap." Beyoncé, then 15 or 16, was making a promotional appearance at a Houston record store. As Tina tried to talk to her, Beyoncé started singing right in her mother's face, movin' and groovin' and letting that voice of hers rip, so Tina, who had never spanked her girls, hauled off and slapped her daughter flush on the cheek. Thwack! The slap reverberated.

"My husband came up and said, `Tina! She's got the No. 1 record on the radio!' I said, `I don't care!' I was terrified. I had seen people take your child and turn them into something you didn't want them to be," Tina says. "I taught my girls to pick up their own suitcases. Pretty is as pretty does. Like my mother said, `You got to be cute on
the inside.' "

Tina worried, however, that her daughter would never forgive her for the slap. "But Beyoncé apologized," she says. "She said she understood why I had done that. And she has never done anything like that again."

These days, mother and daughter still fight, though the issues have changed. Tina is sensitive to B.'s treatment in the tabloids and gets upset that her daughter doesn't do more to publicize her philanthropic ventures, such as donating her salary from her role in Cadillac Records to the Phoenix House in New York City. "Mama, let it go! Why do you read that stuff?" B. tells her. Tina says, "She'll see when she's a mother. I don't want people to think she's a diva."

B.'s costar in Obsessed, Idris Elba, attests that the singer is the furthest thing from a diva. "Actors don't always like to work with musicians. How, for instance, do you tell a worldwide icon like B. that you don't like the way she's doing something?" he asks. "But she was open to anything that helped the work. With one scene, I'd be in her ear before it got going, and it was irritating the fuck out of her. But that helped her walk into the scene blazing."

Nobody becomes a star by accident. The discipline required in the march to mass popularity is equivalent to that of a dozen military campaigns. But at the ancient age of 27, all that's old hat for B. "Being this huge pop star is not my focus right now," she says. "I've done that. I wanted to sell a million records, and I sold a million records. I wanted to go platinum; I went platinum. I've been working nonstop since I was 15. I don't even know how to chill out."

In B.'s version of chillin', she decided to take a "lazy" year in 2008 and work on what would eventually become her third solo record: I Am...Sasha Fierce. Her first two albums had both gone multiplatinum and spawned multiple No. 1 singles such as "Crazy in Love" and "Irreplaceable." The pressure was on. But as she contemplated a new recording, her mother handed her a script. "I said, Please, please, you've got to read this," Tina said. "This is part of our history and it needs to be told."

"You always said you wanted to do something dark," Tina replied. And with that, the die was cast.

Cadillac Records, the fictionalized story of Chicago's Chess Records, introduced B. to what had been for her a lost generation of black musicians who'd ruled the airwaves back in the '50s: Muddy Waters. Little Walter. Willie Dixon. And Etta James. These were her rough-hewn elders, blues and soul division, whose music spoke to their own generation with the same degree of authenticity that rap, which Chuck D. of Public Enemy famously called "the black CNN," did for her peers. For B., the exposure to this history in general, and to the example of Etta James in particular, was to prove a liberation.

Another way to look at Etta James, whose heyday was the '50s and '60s, is as the Sasha Fierce of her era, but a real-life Sasha rather than merely the persona that "takes over" B. when she performs. Etta was Sasha who loved the needle. Sasha who had no place to go when her luck ran out. But a Sasha who never stopped singing.

The rumor circulated recently that B. would no longer answer to Beyoncé. Tina was coming out of the bathroom one evening when she heard the announcer on Entertainment Tonight or one of those shows say that her daughter was dumping her old name. Tina thought: What's this? Her family name is no longer good enough? "I
was in shock," Tina says. "Then they said she wanted to be called Sasha Fierce."

"Her cousin Angie gave her that name," Tina says with relieved laughter. "We'd tease Beyoncé about having a split personality. Me and Angie and Ty, her stylist, would work with her doing a quick change when she'd come off the stage between sets. Beyoncé would start screaming, `What's wrong with you? Where's my shoe?' `Uh oh,' we'd say, `Sasha is here.' I'm like, This is some crazy person who's doing this quick change. She's another person up there. We don't take it personally. Sasha is her bragging side...I don't know whether to say it, but her hip-hop side."

So Sasha provided B. with a land bridge to the complicated country of Etta James. As B. says, "Etta James was not America's sweetheart." Born in 1938, James learned from her mother that she was the offspring od the pool hustler Minnesota Fats. Certainly, from the get-go, she caromed around the world like a cue ball struck off three sides of the table for a trick shot. She started working in clubs at 15. She cursed like a bluesman. She wore a bright orange wig. She ate and drank what she wanted. She shot heroin. She fought addiction. She sang.

"Etta James," B. says, smiling to herself, as one does fondly remembering a rascal. "Etta was bold and unapologetic. Whatever she loved, she did. She said whatever she felt."

During filming, B. turned into a bawdy, foulmouthed ranter, a guiltless creature of impulse. "My mom had to leave the set," she says. "I don't normally curse. Maybe twice a year. I have to be really livid. Somebody saying things or scheduling something where I'm not in control." But for all of the pleasure of wild abandon, she was exhausted from the drama. "It was freeing, but it was also the most emotionally draining time of my life," she says. "Every day I would come home, my eyes would be completely swollen."

The ultimate arbiter of B.'s transformation turned out to be Etta James herself. The singers met at the Fashion Rocks concert last September, where B. also sported for the first time in public a $5 million diamond ring given her by Jay-Z. "I was terrified to meet her," B. says. "Imagine. Having studied her hands, her facial expressions. Like she wasn't real. But she was. I just wanted to please her."

And despite Etta's initial reservations—"I don't think she looks like me," she said upon learning that B. had signed on to play her—she changed her tune after hearing B. perform James' hit from 1961, "At Last."

"Etta told me, `I thought you was a little old thing, girl, but you a brick house!' " B. says, imitating Etta's growl. "She kept saying, `You bad, girl! You bad!' She doesn't say much, but she never tells a lie."

B. had researched the role by visiting with the female addicts in recovery at Phoenix House. "I don't want to be too graphic," she says, "but the women described heroin as orgasmic." Without having done the drug, she suspected she knew how they felt. "On one level, I'm addicted to performing, but I related to their addiction in ways much deeper than that. To play Etta, I used feelings of rejection, neglect, memories of people I loved who passed away. Deep down pain! I felt I needed something as bad as they needed heroin."

"Our lives haven't been picture-perfect," Tina says. "I have a huge family. She's seen addiction up close. Everybody has their pain."

B. grows shy at questions about the objects of her longing. Relationships? She's had so few. She went out with the same boy, a fellow member of
her church, from 12 to 19. Then she feared she'd never find anyone else. "Sometimes you just long for things so bad" is how she wants to leave it.

She refuses to concede that the feeling might have been connected to anything so conventional as the desire to get married. After all, she's a modern girl, self-sufficient. She and Jay-Z had been dating since 2002, if dating means jetting to Cannes to do the same things you could do at the Jersey Shore—i.e., walk down the beach holding hands. Their long-running soap hip-hopera finally ended in New York City last April when they married on the fourth day of the fourth month amid 40 guests and 70,000 or so white orchids flown in from Thailand. B. and her spouse both have IV tattooed on their ring fingers. Who needs wedding bands?

B. says her romantic education convinced her "that the worst thing you can do is press [someone].
Because I'm satisfied with myself, things that are supposed to happen will happen." The trick to love is to "find somebody who makes you
a better person. You are who you're around. And if I don't want to be like you, I don't want to be around you."

She and Jay-Z are both strong-willed people. Not just performers, each also controls entertainment empires worth hundreds of millions of dollars.
Her husband, the former CEO of Def Jam Recordings, stands on the verge of a $150 million partnership with Live Nation. What is the hardest
thing, I ask, about love between two tycoons used to getting their own way?

"It is a power struggle," she admits.
"How do you resolve it?"

"I'm finding out," she says with a laugh. "But if I didn't respect someone and they didn't have that strength, then I would be bored. I wouldn't be attracted to them. But at the same time, I'm not looking for someone to dominate me."

Just now, Tina glides by. Today has been a long shift of manufacturing glamour, and mother and daughter, as easy with each other as girlfriends, are ready to head out. Recently, B. became exasperated babysitting her nephew, who was staying overnight with her and who had claimed, as little
nephews will, that he was allowed to stay up all night. Which he was doing. So at 3 a.m., B. took him to her mother's apartment—they live in the
same building—and dropped the rapscallion off.